Medical devices may be subject to multiple stresses and strains, such as after insertion into a body lumen. Due to the constant stresses and strains placed upon a medical device, microcracks may eventually form on a surface of or within a medical device leading to structural failure and/or patient injury. Increasing the overall strength of a medical device can lead to overrigidity and/or inflexibility, which can unintentionally injure a patient and/or cause further medical complications.
One type of material commonly used for medical devices to attempt to alleviate some of these difficulties is a shape memory alloy. A shape memory alloy has superelastic material characteristics above a transformation temperature and shape memory characteristics below a transformation temperature. Superelastic materials possess unique characteristics that are particularly useful in medical applications. If a piece of a shape memory alloy, such as nitinol, is mechanically stretched, compressed, bent, or twisted in its martensitic phase, it will return to its original configuration upon heating.
The malleable martensitic form of the alloy can be easily deformed and if not constrained, will freely recover upon heating to its original, much stronger austenite phase. In theory, this cycle can be repeated indefinitely. However, microcracks may begin to form through environmental stresses to which the shape memory alloy is subjected. These microcracks can eventually lead to a traumatic structural failure within the shape memory alloy.
Various medical devices incorporating the use of shape memory elements have been known for a number of years. Medical devices adopting the use of shape memory elements rely on the unique structural properties of shape memory alloys in order to achieve their desired effects. The shape memory alloys that are used in stents, for example, retain their new shape when cooled to the martensitic state and are thereafter deformed. However, these same shape memory alloys will recover their original shape when warmed to the austenitic state.